Sound intensity is a function of acoustic pressure and acoustic particle velocity. In measuring sound intensity at a particular location in an acoustic field, it is necessary to determine the instantaneous acoustic pressure and the component of instantaneous acoustic particle velocity in the direction of interest. Once these components are known, any of a number of well known systems can be utilized to calculate intensity. Typical known methods of calculation involve time averaging or spectrum averaging.
For example, if a time averaging system is used, the component of the sound intensity in the direction of interest, I.sub.n can be calculated using the integral equation: ##EQU1## where p=instantaneous acoustic pressure; u.sub.n =component of the instantaneous acoustic particle velocity in the n-direction; T=averaging time; I=sound intensity; dt=derivative over time.
Alternatively, sound intensity in the direction of interest, I.sub.n can be calculated using spectrum averaging from the equation EQU I.sub.n =1/2Re{P(W)U.sub.n *(w)}
where Re {} denotes the real part of the enclosed expression and P(w) and U.sub.n (w) are complex Fourier transforms of the acoustic pressure and component of the acoustic particle velocity in the n-direction respectively. In this equation, U.sub.N *(w) is the complex conjugate of U.sub.n (w).
In the past, three basic types of intensity probes have been used for measuring sound intensity. Such known probes usually utilize a pair of closely spaced microphones or a microphone and an accelerometer for determining acoustic pressure and acoustic particle velocity. However, because the acoustic pressure and particle velocity are not measured at exactly the same point in space, the sound intensity calculated therefrom is in error.
Therefore, a primary objective of the present invention is the provision of a method and means for more accurately calculating sound intensity.
Another objective of the present invention is the provision of a method and means for calculating sound intensity by measuring the acoustic pressure and acoustic particle velocity at the same point in space.
A further objective of the present invention is the provision of a method and means for determining sound intensity wherein acoustic particle velocity and acoustic pressure are measured directly.
These and other objectives will become apparent from the following description of the invention. cl SUMMARY OF THE INVENTION
The present invention relates to a method and means of determining sound intensity by measuring the instantaneous acoustic particle velocity and instantaneous acoustic pressure at a single point. Sound intensity can then be calculated from these two components.
The acoustic pressure is measured by subjecting a microphone to the sound such that the microphone diaphragm vibrates. A microphone output voltage is generated by the vibrating diaphragm and is proportional to the instantaneous acoustic pressure.
The instantaneous acoustic particle velocity is measured by projecting a laser beam onto the vibrating diaphragm and reflecting the beam from the diaphragm so that the Doppler frequency shift of the beam can be detected. A laser Doppler vibrometer is used to project the beam and detect the Doppler frequency shift of the beam. The Doppler frequency shift of the beam is then converted by a Doppler frequency tracker into a tracker output voltage which is proportional to the instantaneous acoustic particle velocity. The sound intensity is calculated from the voltages by employing any well-known average technique, such as time averaging or spectrum averaging.